1. Field of the Invention
This invention relates to threshold voltage modulation in FET semiconductor devices.
2. Brief Description of the Prior Art
The threshold voltage, V.sub.T, of a field effect transistor (FET) depends upon the voltage on the substrate or body region (the region between the source region and the drain region). The threshold voltage can be changed by changing the body voltage. For an n-channel transistor, an increase in body voltage lowers (makes less positive or more negative) the threshold voltage. For a p-channel transistor, an increase in the body voltage raises (makes more negative or less positive) the threshold voltage, V.sub.T. Accordingly, for an n-channel transistor, when the gate voltage goes high, the capacitive coupling to the body region raises the body voltage and lowers the threshold voltage. This provides increased drive current. When the gate is turned off, the gate coupling raises the threshold voltage by lowering the body voltage, thereby providing decreased leakage current when the transistor is off. This capacitive coupling of the gate to the body is significant only when the conductive channel (inversion region) from the source region to the drain region is not formed. When the conductive channel is formed, it electrically isolates the gate from the body. Therefore, after the channel is formed, the advantage of the capacitive coupling of the gate to the body is no longer improved.
In the prior art, the edges of a transistor were sometimes doped more heavily to prevent leakage at the edge. The more heavily doped edge region would have a higher V.sub.T and thus the gate-to-body capacitive coupling would exist at the edge region for a greater range of gate voltage than at the channel region. However, the more heavily doped region would be made as small as practical, and a sidewall dielectric thicker than the gate oxide would generally be used. A sidewall insulator is required with mesa isolation. With shallow trench isolation (STI), the space between sidewalls is filled with dielectric. In the prior art, the sidewall insulator is typically several time the thickness of the gate oxide. Thus, the contribution of the highly doped edge region to the gate-to-body coupling would not be substantial.
Assuming, for example, an n-channel transistor, coupling of the gate to the body of the transistor causes dynamic V.sub.T modulation, thereby increasing V.sub.T when the transistor is off and decreasing V.sub.T when the transistor is on. However, direct coupling from the gate to a body contact results in gate current and diminished chip area. These problems have been attacked in the prior art by providing separate capacitors which are connected between the gate and the body of the transistor. This eliminates gate current but still diminishes chip area. Also, resistance in the connection of the capacitor to the body diminishes the effectiveness of the coupling. It is therefore desirable to increase the capacitive coupling between the gate and the body of a FET as well as to do so in a more economical, more effective and simpler procedure.